1. The Field of the Invention
This invention pertains to building construction, and, more particularly, to novel methods and apparatus for anchoring building walls to foundations and lower floors thereof. The invention provides an automatic adjusting mechanism to remove slack in a hold-down system caused by wood shrinkage over time or wood crushing caused by earthquakes.
2. The Background Art
Strong winds and earthquakes subject walls and others elements of a building to tremendous forces. If these forces are not distributed to the proper elements or structures capable of withstanding such forces, the building may be torn apart. Foundations are often the strongest element of a building. Securely tying the walls of a building to the foundation greatly improves structural performance during periods of strong wind or earthquake. Securement promotes single-body motion and limits whiplash amplification that often results in structural failure.
Under such extreme conditions, a building may be violently loaded or shaken both vertically and back and forth in a lateral (side to side) direction. If a shear wall is tightly restrained at its base, loads may be smoothly transferred to the foundation. The loads may then be resolved in the foundation, where they appear as tension and compression forces.
Buildings are often composed of long walls, (walls with a length greater than the height) and short walls (walls that have a length shorter than the height). The tendency for a wall to lift vertically off a foundation is inversely proportional to the length of the wall. Tall, narrow shear walls, which may be found in nearly all homes, act as lever arms and may magnify an imposed load. In certain instances, the actual load on the securement system may be magnified to several times the originally imposed load.
Wall securement may reduce or prevent lateral and vertical motion between the walls and the foundation. Additionally, it may be necessary to support the wall against shear forces that would tend to distort the wall's general rectangular shape. Building codes often require external and load bearing walls to be shear resistant by providing a plywood plane to support shear forces that may be imposed on the wall. Many times, building codes also require lateral and vertical securement of a wall to the foundation. Lateral and vertical securement may be accomplished by employing hold-downs, also referred to as tie-downs.
Wood products change in dimension as moisture content changes. Accordingly, floor systems using solid sawn joists typically shrink approximately five percent across the grain. Under certain conditions, floor systems have been known to shrink six and one-half percent within a year. This shrinkage is typically part of the overall process and condition called “settling,” which may also include other changes such as settling of foundations.
In typical practice, hold-down systems may include a threaded rod or an anchored strap capturing the base plate or sill plate of a wall (i.e., the bottom, horizontal member above which the studs extend vertically). Over time (e.g., months, years) wood loses moisture, shrinks, and the building settles. Threaded-rod-type anchors become loose. Strap-type anchors buckle, if positively engaged, and become loaded in compression, or the like. Moreover, under a dynamic load, such as an earthquake or high wind conditions, wood crushes or collapses. Thus, gaps permitting undesirable motion of the walls with respect to the foundation may continue to increase during a catastrophic event.
Accordingly, current hold-down systems do not provide a solution for wood shrinkage, wood crushing, and other settling issues. As a result, with the passage of time, the walls of a building may be able to move significant distances before engaging the hold-down systems designed to secure them in place. This delay in engagement can result in a fifty percent to seventy percent loss in lateral, load-bearing capacity. Neither shrinkage nor crushing are well accommodated or otherwise resolved in currently available systems. These problems lead to a significant reduction in the lateral, load-bearing capacity of shearwalls.
What is needed is a hold-down or tie-down system accommodating shrinkage of building materials. Such a hold-down system may significantly improve the strength of shear walls subject to shrinkage of constituent materials.